Nintendo confirmed today that it has sold five million of its 3DS portable game players in the U.S. since the device went on sale 15 months ago.

Based on Nintendo’s previous sales figures, it shows that demand has been on the decline for the hardware since the beginning of the year.

Here’s the math:

If Nintendo said in January that it sold four million 3DS devices in the U.S. last year, that means it has sold only one million devices so far this year. Since the device didn’t go on sale until April 2011, it sold about 500,000 units every month for eight months last year. In contrast, it has sold one million units, at an average of 167,000 a month, for the past six months.

In June, it disclosed that it sold 155,000 Nintendo 3DS systems and 150,000 of its original DS systems. It sold 95,000 Wii consoles.

But Nintendo is no stranger to the cyclical nature of the games business, and continues to bank on upcoming hardware releases to spur additional sales.

Last month, the company announced the Nintendo 3DS XL, which will be available on Aug. 19 for $199.99. The handheld gaming device features screens that are 90 percent larger than the previous 3DS. Both devices allow users to view images in 3-D without the goofy glasses. It will also launch the New Super Mario Bros. 2 on Aug. 19, and says its holiday lineup will include hand-held debuts for both Luigi’s Mansion and Paper Mario.

Later this year it will release the Wii U, the company’s next-generation console.

The good news to take away from today’s announcement is that there are now five million 3DS owners in the wild, and they will likely continue to purchase new games. In addition, there are still many people who own the original DS. In October, Nintendo said it had sold a total of 50 million portable gaming systems in the U.S.

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